Simplify the following expression and state the conditions under which the simplification is valid. You can assume that $t \neq 0$. $x = \dfrac{t^2 + t - 20}{3t + 9} \times \dfrac{-4t - 12}{t^2 - 4t} $
First factor the quadratic. $x = \dfrac{(t - 4)(t + 5)}{3t + 9} \times \dfrac{-4t - 12}{t^2 - 4t} $ Then factor out any other terms. $x = \dfrac{(t - 4)(t + 5)}{3(t + 3)} \times \dfrac{-4(t + 3)}{t(t - 4)} $ Then multiply the two numerators and multiply the two denominators. $x = \dfrac{ (t - 4)(t + 5) \times -4(t + 3) } { 3(t + 3) \times t(t - 4) } $ $x = \dfrac{ -4(t - 4)(t + 5)(t + 3)}{ 3t(t + 3)(t - 4)} $ Notice that $(t + 3)$ and $(t - 4)$ appear in both the numerator and denominator so we can cancel them. $x = \dfrac{ -4\cancel{(t - 4)}(t + 5)(t + 3)}{ 3t(t + 3)\cancel{(t - 4)}} $ We are dividing by $t - 4$ , so $t - 4 \neq 0$ Therefore, $t \neq 4$ $x = \dfrac{ -4\cancel{(t - 4)}(t + 5)\cancel{(t + 3)}}{ 3t\cancel{(t + 3)}\cancel{(t - 4)}} $ We are dividing by $t + 3$ , so $t + 3 \neq 0$ Therefore, $t \neq -3$ $x = \dfrac{-4(t + 5)}{3t} ; \space t \neq 4 ; \space t \neq -3 $